1. Field of the Invention
The present invention relates to a backlight unit of a liquid crystal display (LCD), and more particularly to a backlight unit of a liquid crystal display, which has a structure commonly applicable to any size of LCDs, and can be structured with a smaller number of light emitting diodes.
2. Description of the Related Art
A light emitting diode (LED) is a semiconductor light emitting device, which emits various colors of light, with light source being constituted by compound semiconductors made of various materials, such as GaAs, AlGaAs, GaN, InGaN and AlGaInP. With the advent of highly efficient three primary colors (red, blue, green) and white LEDs realized by a nitride having excellent physical and chemical properties, the LEDs have more diverse applications. For instance, the LEDs are used in various applications, such as a backlight of a keypad or a liquid crystal display, a signal lamp, a guide lamp at a landing strip of an airport, a highly directional reading lamp in an airplane or a vehicle, street lighting, and the like.
As a standard for determining characteristics of the LEDs, colors, brightness, intensity of light, and the like, of the LEDs can, be used, and these are determined primarily by the material of the compound semiconductor for the LEDs, and secondarily by a package structure for mounting LED chips.
FIGS. 1a and 1b are a perspective view and a section side elevation illustrating an LED device used as a light source for a conventional backlight.
Referring to FIGS. 1a and 1b, an LED device 10 comprises a lead line 12 for inputting or outputting an electrical signal, a frame 11 made of a plastic material and being provided with a thermally conductive material therein, an LED chip 14 mounted on the frame 11, and a lens 16 coupled to the frame 11 at the top surface of the frame 11 while being structured such that light generated from the LED chip 14 can be reflected to the horizontal direction.
The lens 16 has a pyramidal shape attached on a semi-spherical shape, and acts to refract light emitted in any direction from the LED chip 14 located at the center of the top surface on the frame 11 to the horizontal direction.
The package structure consisting of the frame 11 and the lens 16 is structured such that the light generated from the LED chip 14 is emitted approximately in the horizontal direction while heat generated from the LED chip 14 is dissipated to the outside, thereby providing stable operation characteristics to the LED device 10.
The above LED device 10 is a spot light source. Thus, in order to realize surface emission with the spot light sources, a separate structure is required. With regard to this, a structure, wherein an LED array with a plurality of LED devices 10 arranged in a line therein is mounted at the center of a reflection plate having a semi-cylindrical shape, thereby realizing the surface emission through the reflection plate, has been conventionally provided.
FIG. 2 shows a conventional backlight unit using the LED device 10 having the horizontal light emitting characteristics as described above, and FIG. 3 is an enlarged view of an LED array 21 provided in the backlight unit shown in FIG. 2.
As shown in FIGS. 2 and 3, the backlight unit 30 comprises: an LED array module 20 including a plurality of LED devices 10, each of which is packaged as shown in FIG. 1 while emitting one color among red, blue and green colors; a printed circuit board (PCB) 21 with printed circuit patterns for transmitting an electrical signal formed thereon and for mounting the plurality of LED devices 10 in a line thereon such that three colors can be alternately emitted; and a reflection plate 31 having a semi-cylindrical shape, in which the LED array module 20 is located at the center of the reflection plate 31. The PCB 30 of the array module 21 is preferably made of Al, which has good thermal conductivity in order to enhance heat dissipation efficiency.
The backlight unit 30 realizes surface emission over all ranges of an LCD screen by totally reflecting the light, emitted from the respective LED devices mounted on the LED array module 20 in the horizontal direction, in the perpendicular direction through the reflection plate 31.
The number of the LED array modules 20 required for the backlight unit having the structure as described above is increased in proportion to the screen size of the LCD display, which uses the LED array modules. Further, the number of the LED devices 10 required for each of the LED array modules 20 is also increased in proportion to the screen size of the LCD display using the LED array modules.
As described above, the conventional backlight unit has problems in that the LED array module 21 and the reflection plate 23 must be redesigned according to the screen size of the LCD display, and in that the number of the expensive LED devices 10 is increased in proportion to the screen size of the LCD display.
Moreover, since the independently packaged red, blue and green LED devices are mounted on the PCB 21 in the conventional backlight unit, power must be separately applied to the respective red, blue and green LED devices, and three colors of the light emitted from the LED devices 10 are mixed at the outside. Thus, in order to adequately mix the three colors emitted from the LED devices 10, it is necessary to provide a space for mixing, thereby providing a thick LCD display.
Further, since the plurality of LED devices 10 are linearly arranged to form a linear emitting source, length of the LED array module is determined based on the screen size of the LCD. Accordingly, as the screen size of the LCD is increased, not only is the number of the LED devices 10 for one LED array module 20 increased, but the size of the expensive aluminum PCB 21 is also increased. Thus, there are problems in that the conventional LCD has inefficiency in view of manufacturing costs, and in that as additional PCBs 21 must be provided according to the screen size of the LCD, the LED devices 10 are not commonly applicable.
Further, the conventional LED device 10 refracts light in the horizontal direction using difference of the refraction rates of the lens 13 and other media. Accordingly, since the light generated from the center of the LED in a direction to an upper portion of the LED is transmitted without having its direction changed, there can occur a hot spot at the center of the LED device, thereby requiring a shadow sheet for the backlight unit 30 in order to prevent the hot spot from being generated. Additionally, since the conventional backlight unit adopts a 1-chip per 1-package structure wherein the LED device packages one chip, it is not price competitive.